Heretofore various systems have been studied and developed as communication systems. As one of them there is known the spread spectrum communication system (hereinbelow called simply SS communication system).
In this SS communication system, on the transmitter side, a signal such as narrow band data, voice, etc. is spread in a wide band spectrum by means of a pseudo noise code (PN code) to be transmitted and on the receiver side the signal is reproduced by inversely spreading the wide band signal in the original narrow band signal by means of a correlator.
It is known that this SS communication system is resistant to external interference and noise and in particular with respect to a narrow band disturbing wave, e.g. to a continuous wave (CW) it has an excluding power of process gain (PG). The process gain can be given by a following formula; ##EQU1## where the radio frequency band width described above means the band width of the transmitted SS signal and the information speed is a data speed in the base band channel.
A receiver section of such a communication device using the SS communication system (SS communication device) is disclosed in U.S. Pat. No. 4,926,440. In the receiver section of this SS communication device, as indicated in FIGS. 1A and 1B of the USP stated above, data are restored through a BPF (band pass filter), an amplifier, an envelope detector, a comparator circuit and a pulse width enlarging circuit connected with the output of a surface acoustic wave (hereinbelow abbreviated to SAW) convolver. Output waveforms of different parts are as indicated in FIG. 3 of the USP stated above.
Here, in an SS communication device having the construction described above, when a received signal, in which narrow band disturbing waves, e.g. CW, are mixed in the band of the SS signal, enters the correlator, the waveform at the output of the correlator is a waveform, in which a convolution integration result of the SS signal in the received signal and the SS signal in the reference signal, i.e. an envelope of correlation spikes SP.sub.1, and a convolution integration result of CW in the received signal and the reference signal, i.e. an envelope of spurious noise SP.sub.2, are superposed on each other, as indicated in FIG. 5(a). This remains unchanged, as indicated in FIG. 5(b), even after the correlation output when variations are produced in such correlation spikes and spurious noise is envelope-detected.
In such a state, in the case where a reference voltage is set and the correlation spikes are converted into binary values by means of the comparator circuit indicated in FIG. 6 in the USP stated above, the correlation spikes and the spurious noise cannot be separated. For this reason, erroneous judgment can take place that there are no correlation spikes, although there are (b.sub.1 .about.b.sub.n), as indicated in FIG. 5(c) in the USP stated above, or that there are correlation spikes, although there are not (a.sub.1 .about.a.sub.n). In this way, in the case where signals obtained by shaping waveform of such correlation spikes (correlation pulses) converted into binary values by means of a pulse width enlarging circuit are used for reproducing data, erroneous data are reproduced, depending on sample points S.sub.1 .about.S.sub.3, as indicated in FIG. 5(d), which worsens the error rate.
For this reason, a receiver by the system indicated in FIGS. 9A and 9B in the USP stated above has been proposed in order to remove this drawback in the prior art technique described above.
By this system, the output of the pulse width enlarging circuit is integrated through an LPF (low pass filter) and a signal thus integrated is shaped in the waveform by a waveform shaping circuit to reproduce the data. However, also in this case, erroneous data are reproduced similarly to the preceding case, if there are many erroneous judgments in the comparator circuit, as indicated in FIG. 5(c) stated above.
FIG. 5(a) indicates a case where, when correlation spikes are superposed on spurious noise in a time region, the phase relation of the correlation spikes with respect to the spurious noise at the peak point of the correlation spikes is in a region of -.pi./2.about..pi./2. However it is also conceivable that they are superposed on each other in a region of .pi./2.about.3.pi./2.
In the case of the region of -.pi./2.about..pi./2, the correlation spikes are superposed on the spurious noise in the direction, in which the sum thereof is additive, and the maximum is produced at the same phase (0), as indicated in FIG. 6(a), the level thereof being equal to the level of the spurious noise+the level of the correlation spikes.
In the case of the region of .pi./2.about.3.pi./2, the correlation spikes are superposed on the spurious noise in the direction, in which the sum thereof is subtractive, and the minimum is produced at the opposite phase (.pi.), as indicated in FIG. 6(b), the level thereof being equal to the level of the spurious noise-the level of the correlation spikes.
As described above, when the correlation spikes are superposed on the spurious noise, by the comparator circuit by the system indicated in FIG. 6 of the USP stated above, only the case of superposition in the region of -.pi./2.about..pi./2 can be detected, which can be a cause of worsening of the data reproduction.